Superlattice pattern of standing waves on a fluid surface. When a fluid layer is excited by vertical vibration of the container, the flat surface is unstable and a variety of standing wave patterns arise. In this example, *two* interacting hexagonal lattices of standing waves (each one like a honeycomb of small hexagonal cells) are superimposed at an angle of about 22 degrees. The net result is a pattern with triangular overall symmetry, and apparent structure on a larger scale. In this color image, the red regions are tilted most strongly from the horizontal. The pattern is stabilized by nonlinear interactions between standing waves propagating in different directions. This is one of many distinct regular patterns that are found in nonlinear systems. Courtesy: A. Kudrolli, B. Pier, and J.P. Gollub (to appear in Physica D).
